Process for the production of cumene

ABSTRACT

Production of cumene by alkylation of benzene with propylene on a solid phosphoric acid catalyst, using in a first step a plurality of catalyst beds in series and in a second step a single catalyst bed, using an overall benzene/propylene molar ratio of at least 5:1, continuously delivering the benzene to the first bed, continuously delivering a series of streams of liquid propylene respectively to the first bed and, in the form of a cold stream, between each pair of contiguous beds, and carrying out a partial recycle of reaction products of the second step as a cold stream between the first and the second step, and using a benzene/propylene molar ratio higher than 13:1 at the inlet of each bed of the first step, and higher than 25:1 at the inlet of the bed of the second step.

The present invention relates to improvements in the process for theproduction of cumene from benzene and propylene on solid phosphoric acidcatalysts.

Cumene (or isopropylbenzene) is a valuable product, generally used as anintermediate in the synthesis of other chemical compounds, especially inthe production of phenol or acetone via cumene hydroperoxide.

In the known art, the alkylation of benzene with propylene is carriedout either in the gaseous or in the liquid phase in the presence ofsuitable catalysts, as described by S. H. Mc.Allister et al. in"Chemical Engineering Progress", Vol. 43, No. 4 (1947), at pages189-196. The catalysts generally used are based on solid phosphoricacid, described for example in British Patent No. 836,539 and U.S. Pat.No. 2,120,702 and 2,613,188, as well as in the article mentioned above.

In the known industrial processes, liquid benzene and liquid propyleneare usually fed to a reactor which contains the solid catalyst in theform of a fixed bed or in the form of a series of fixed beds. In orderto minimize the formation of poly-isopropyl-benzene by-products(di-isopropylbenzene and tri-isopropylbenzene) an excess of benzene withrespect to the propylene is usually maintained in the feed, thus toreduce also the quantity of oligomers of propylene, such as dimers andtrimers.

The formation of said by-products is promoted by the formation of hotpoints in the catalytic bed, as a result of the exothermal nature of thealkylation reaction. In said hot points a gaseous phase may form, inwhich the concentration of propylene is high. Attempts have been made toovercome this drawback by arranging the catalyst in the form of a seriesof fixed beds and feeding partly the cold reagents (benzene andpropylene) between each pair of contiguous beds, as described, forexample, in U.S. Pat. No. 3,813,451.

This expedient certainly makes easier the control of the exothermalnature of the reaction, but does not permit the best ratio betweenbenzene and propylene to be maintained in the reaction medium. As aresult, the quantity of by-products, especially polyalkylbenzenes, isrelatively high.

It has now been found that the drawbacks of the known art can beovercome, and that it is possible to produce cumene from benzene andpropylene by operating in the liquid phase and on solid phosphoric acidcatalysts, by means of a simple and convenient process in which theconversion into polyalkylbenzenes and propylene oligomers is reduced tovery low values.

More particularly, the present invention provides a continuous processfor the production of cumene by alkylation of benzene with propylene ona solid phosphoric acid catalyst, said process being characterized by:

using two reaction steps in series, the first step being carried out ina plurality of catalytic beds in series and the second step in acatalytic bed of volume equal or similar to the overall volume of thecatalytic beds of the first step;

operating in said first and second steps in the liquid phase, at atemperature of from 170° to 270° C. and under a pressure of from 30 to50 Kg/sq.cm.;

using an overall benzene/propylene molar ratio of at least 5:1,continuously delivering the whole of the liquid benzene to the firstcatalytic bed of the first step, continuously delivering a series ofstreams of liquid propylene to the first catalytic bed of the first stepand, in the form of a cold stream, between each pair of contiguous bedsin the first and second steps and carrying out a partial recycle offinal reaction products as a cold stream between the first and thesecond step, in such amounts that the benzene/propylene molar ratio behigher than about 13:1 at the inlet of each individual catalytic bed ofthe first step, and higher than about 25:1 at the inlet of the catalyticbed of the second step;

recovering cumene from the reaction products discharged from the secondstep.

The present invention is essentially based on the finding that the moreuniform the composition and temperature of the reaction medium, the morefavorable the course of the catalytic alkylation reaction of benzenewith propylene as regards the formation of by-products. This isespecially true in the final step when the concentration of cumene inthe reaction mixture is higher.

Conditions similar to the optimum conditions indicated above areachieved according to the present invention by delivering decreasingportions of the overall amount of propylene upstream of each catalyticbed and by rehomogenizing the reaction medium by means of a partialrecycle of liquid reaction products of the second step in which thereaction is brought to completion. By using this expedient the variationin concentration of free propylene is low along each catalytic bed, asis also the gradient of temperature, the latter being on an average ofthe order of 20° C. along the catalytic beds of the first step, and ofthe order of 10° C. along the catalytic bed of the second step.

It has also been ascertained that the use of relatively high residencetimes has a beneficial influence, in particular in the final reactionstep in which the conversion of the free propylene present in thereaction medium is substantially brought to completion.

Therefore, the residence time in the completion step (second step) ispreferably similar to that in the reaction step proper (first step).

Finally, by delivering the whole of the benzene to the first catalyticbed and by carrying out a recycle of liquid reaction products betweenthe two steps, the molar ratio between benzene and propylene ismaintained at the highest possible values in each point of the reactionsystem, with consequent advantages deriving from the low formation ofby-products.

Therefore, the process of the present invention is carried out in tworeaction steps in series, using in the first of said steps a pluralityof separate catalytic beds in series. The number of beds may generallyrange from 2 to 5, but preferably there are used three catalytic bedshaving the same or about the same volume.

Moreover, a characteristic of the present invention consists in using inthe second step a single catalytic bed of volume equal, or about equalto the overall volume of the catalytic beds of the first step.

The alkylation catalysts suitable for the purpose are solid catalystsbased on phosphoric acids, which are generally prepared by mixing acidsin which phosphorus has a valency of 5, with a silicic material andtreating the resulting mixture at elevated temperature. A preferredcatalyst for the purposes of the present invention is prepared fromdiatomite having a silica content higher than 70% by weight. Inparticular, this support is heated at a temperature of the order of 250°C., until a specific surface area of 10-30 sq.m/g is achieved. Thesupport is then mixed with the commercial complex product known aspolyphosphoric acid, having a P₂ O₅ content of the order of 85% byweight. Conveniently, one part by weight of support is mixed with twoparts by weight of polyphosphoric acid, operating at ambient temperatureor slightly above. After homogenization the mixture is formed into smallcylinders, which typically have a diameter and a height of the order of5-6 mm. The cylinders are then calcined at a temperature of the order of300° C. for a period of 90-150 minutes.

The catalyst is arranged in the form of a fixed bed in reactors,generally tubular, according to the configuration already indicated.

In carrying out the process of the present invention, the whole of thebenzene is delivered to the first catalytic bed of the first reactionstep, whereas the propylene feed is divided into a plurality of streams,which are respectively delivered in the liquid form to the inlet of eachindividual catalytic bed. In each case the overall molar ratio betweenbenzene and propylene in the feed is maintained at a value of at least5:1. The maximum value of said ratio is not critical, but it is notconvenient to exceed a limit of 10:1. The preferred values for saidratio are of the order of 7:1-8:1.

The division of the propylene feed between the catalytic beds of thefirst step is regulated in such a way that the benzene/propylene molarratio at the inlet of each individual bed be higher than about 13:1, andpreferably higher than 20:1, up to a maximum value of 25:1. The molarratio between benzene and propylene at the inlet of the catalytic bed ofthe second step depends on the quantity of propylene fed therein, and onthe importance of the recycle of liquid reaction products, the latteracting additionally as a cooling liquid, as will be shown hereafter. Ineach case the conditions are regulated in such a way that the molarratio between benzene and propylene at the inlet of the second step behigher than about 25:1, and preferably of the order of 40:1. The maximumvalue of said ratio is not critical and is essentially dictated byeconomical considerations.

As regards the feed of propylene, said compound may be pure orsubstantially pure, or may contain inert compounds, such ashydrocarbons, non-reactive under the operating conditions, in amountsnot exceeding about 15% by weight.

The reaction temperature is generally from 170° to 270° C. and theoperating pressure is such as to maintain the reaction medium in theliquid phase. The pressure is generally from 30 to 50 kg/sq.cm.

The exothermal nature of the reaction is controlled by setting asuitable inlet temperature to the first step, and by means of theinjection of cold propylene stream (15°-50° C.) between each pair ofcontiguous catalytic beds, and in the second step, by means of theadditional recycle of a fraction of the reaction products cooled down totemperatures of the order of 30°-100° C. The reaction products recycledto the second step may be those discharged from the second step as theyare, or may be freed from the heavier fractions. Thus, depending oneconomical considerations, these recycle reaction products maypreviously be freed from the by-products, and possible also from afraction or substantially the whole of the cumene present therein torecycle a stream consisting essentially of unreacted benzene andpossibly of propane. This recycle permits the benzene/propylene ratio inthe reaction mixture at the inlet of the second step, and the inlettemperature of said mixture, to be controlled.

The space velocity, expressed as volume of liquid per volume of catalystand per hour, is conveniently of the order of 0.3-1.4, preferably of theorder of 0.4-0.9.

According to a typical embodiment of the process of the presentinvention, there is used an overall molar ratio between benzene andpropylene of 7:1, operating with an inlet temperature of the reagents tothe first catalytic bed of the order of 190°-205° C., with an inlettemperature of the mixture to the second step of the order of 220°-230°C. and under the other conditions specified above.

In this manner the conversion with respect to propylene is typicallyhigher than 80%, reckoned at the outlet of each individual catalytic bedof the first step, and the conversion of propylene at the outlet of thesecond step is complete or substantially complete. Moreover, theincreases in temperature are of the order of 15°-25° C. in the catalyticbeds of the first step and of the order of 10°-15° C. in the catalyticbed of the second step.

Under these conditions the formation of by-products is of the order of30-33 parts by weight for each 1000 parts by weight of cumene.

The separation of cumene from the reaction products discharged from thesecond step is carried out by conventional methods, such asdistillation. The benzene recovered may be recycled and delivered to thefirst catalytic bed together with fresh benzene.

The following experimental example is illustrative and non-limitativefor the invention.

EXAMPLE

There is used a solid phosphoric acid catalyst in the form of smallcylinders with a size of 5.5 mm, obtained as described above in respectof the preferred embodiment of the preparation of the catalyst. Thiscatalyst is arranged in the form of fixed beds in the first and thesecond reaction steps.

More particularly, with reference to the accompanying drawing, there areused a first step reactor 11 and a second step reactor 12. The catalystis arranged in the reactor 11 in the form of three separate fixed bedsin series shown in 18, 19 and 20. The beds 18, 19 and 20 have a similarvolume of the order of 25-30 m³. In reactor 12 the catalyst is arrangedfor the sake of convenience in the form of three beds 21a, 21b and 21c.However, owing to the absence of intermediate feedings, the functioningis that of a single bed having a volume equal to the overall volume ofbeds 18, 19 and 20 of the first step.

Fresh benzene is circulated through pipe 22 and mixed with the recyclebenzene stream delivered through pipe 23. The weight ratio between thetwo streams is about 6:1. The two streams are combined and deliveredthrough pipe 24 at a rate of 75,180 kg/h. The titre in benzene of saidstream is 96% by weight, the remaining percentage consisting essentiallyof cumene and non-aromatic compounds. Liquid propylene is delivered at atemperature of 30° C. through pipe 25 at a rate of 6,060 kg/h. Thisstream has a titre in propylene of about 92% by weight, the remainingpercentage consisting essentially of propane.

The propylene stream of pipe 25 is delivered partially to reactor 11(5,195 kg/h) through pipe 26, and the remaining part to reactor 12 (866kg/h) through pipe 29.

The stream of pipe 26 is in turn divided into three steams which aredelivered respectively through pipe 24 ahead of the catalytic bed 18,through pipe 28 ahead of the catalytic bed 19, and through pipe 27 aheadof catalytic bed 20. More particularly, propylene is delivered throughpipe 24 at a rate of 2,000 kg/h to the inlet of catalytic bed 18, uponmixing with fresh benzene and recycle benzene and pre-heating of thecombined streams. Water is also added to this stream in a quantity of 42kg/h. The propylene streams of pipes 28 and 27 are delivered at a rateof 1,732 kg/h and respectively 1,463 kg/h.

The propylene stream of pipe 29 is mixed with the products issuing fromreactor 11 through pipe 30.

The resulting mixture circulated through pipe 31 is mixed with therecycle products circulating through pipe 32 at a rate of 14,750 kg/hand at a temperature of about 47° C., and the whole is delivered toreactor 12 through pipe 33. The reaction products are discharged fromreactor 12 through pipe 34. A part of said products is cooled inexchanger 36 and recycled in the manner indicated above. Reactors 11 and12 are operated at a pressure of about 35-40 kg/sq.cm, thereby tomaintain the reaction medium in the liquid phase. Under theseconditions, the course of the alkylation reaction in the differentcatalytic beds is on an average the following:

catalytic bed 18: benzene/propylene molar ratio at the inlet of about22:1, inlet temperature of 205° C., outlet temperature of 227° C., molarconversion of propylene of 86.2% and molar selectivity of cumene of 98%with respect to the converted propylene;

catalytic bed 19: benzene/propylene molar ratio at the inlet of about21:1, inlet temperature of 221.5° C., outlet temperature of 243.5° C.,overall molar conversion of propylene of 92.2% and overall molarselectivity of cumene of 95.7% with respect to the converted propylene;

catalytic bed 20: benzene/propylene molar ratio at the inlet of about23:1; inlet temperature of 238.5° C., outlet temperature of 256° C.,overall molar conversion of propylene of 94.8% and overall molarselectivity of cumene of 94.3% with respect to the converted propylene;

catalytic bed 21a-21b-21c: benzene/propylene molar ratio at the inlet ofabout 40:1, inlet temperature of 227° C., outlet temperature of 236.5°C., overall molar conversion of propylene of 99.9% and molar selectivityof cumene of 94.7% with respect to the converted propylene.

The reaction products are submitted to suitable treatments to separatethe various constituents. More particularly, said products are deliveredthrough pipe 35 to apparatus 13 in which they are expanded withconsequent evaporation of about 21% by weight of the liquid. Thesevapors issuing through pipe 37 are cooled and condensed, and thehydration water of the catalyst is removed in decanter 14 and deliveredcontinuously to the alkylation together with the reagents.

The organic phase is delivered through pipe 39 to apparatus 15, in whichpropane is removed at the top through pipe 41 at a rate of 505 kg/h.

At the bottom of apparatus 15 there is recovered a liquid stream (16,575kg/h) consisting essentially of benzene with small amounts of cumene andnon-aromatic compounds, which is recycled through pipe 42, excepting asmall percentage discharged to keep the content of non-aromaticcompounds at low values. The liquid products discharged from the bottomof apparatus 13 are delivered through pipe 38 to apparatus 16, in whichimpure benzene is recovered at the top at a rate of 48,375 kg/h and,excepting a vented portion, is recycled through pipes 43 and 23 togetherwith the benzene recycled through pipe 42.

The bottom liquid products of apparatus 16 are delivered through pipe 44to apparatus 17, in which cumene is discharged at the top with a titreof 99.8% by weight and at a rate of 15,215 kg/h, and is recoveredthrough pipe 45. At the bottom of the column the heavy by-products aredischarged at a rate of 470 kg/h through pipe 46. The quantity of saidby-products is thus equal to 31 kg for each 1000 kg of cumene.

We claim:
 1. A continuous process for the production of cumene byalkylation of benzene with propylene on a solid phosphoric acidcatalyst, comprising the steps of:providing a two reaction step serieswherein a first step is comprised of a plurality of catalyst beds influid connection in series and a second step in fluid connection withsaid first step comprised of a catalyst bed having a volumesubstantially equal to the overall volume of the catalyst beds of thefirst step; providing an amount of liquid benzene and liquid propyleneto the two reaction step series wherein the benzene/propylene molarratio is at least 5:1; operating the two reaction step series in theliquid phase, at a temperature of from 170° C. to 270° C., at a pressureof 30 to 50 Kg/sq. cm. wherein the whole of the benzene in liquid formis continuously delivered to a first catalyst bed of the first step, anda series of streams of the liquid propylene at a temperature of 15°-50°C. are continuously delivered respectively to the first catalyst bed ofthe first step and between each pair of contiguous bed in the first andsecond steps; carrying out a partial recycle of reaction products of thesecond step whereby a cold stream of the reaction product of the secondstep is cooled to a temperature of 30° to 100° C. and recycled betweenthe first and the second step in an amount such that thebenzene/propylene molar ratios at an inlet of the catalyst bed of thesecond step is higher than about 25:1; and recovering cumene fromreaction products discharged from the second step.
 2. The process ofclaim 1, wherein from 2 to 5 catalyst beds are used in the first step.3. The process of claim 1, wherein three catalyst beds are used in thefirst step.
 4. The process of claim 1, wherein the overallbenzene/propylene molar ratio is from 5:1 to 10:1.
 5. The process ofclaim 1, wherein said overall benzene/propylene molar ratio is from 7:1to 8:1.
 6. The process of claim 1, wherein the benzene/propylene molarratio at the inlet of each individual catalyst bed of the first step isfrom 20:1 to 25:1.
 7. The process of claim 1, wherein thebenzene/propylene molar ratio at the inlet of the second step is of theorder of 40:1.
 8. The process of claim 1, wherein said recycle reactionproducts are previously substantially freed from components having aboiling point higher than that of benzene.
 9. The process of claim 1,wherein there is used a space velocity of from 0.3 to 1.4 volumes ofliquid reaction medium per volume of catalyst and per hour.
 10. Theprocess of claim 9, wherein said space velocity is from 0.4 to 0.9. 11.The process of claim 1, wherein the residence time in the second step issubstantially equal to the overall residence time in the first step. 12.The process of claim 1, wherein the increase in temperature along eachindividual bed is controlled to a value of from about 15° to about 25°C. in the first step, and to a value of from about 10° to about 15° C.in the second step.